Journal of Vertebrate Paleontology 20(1):197–200, March 2000 q 2000 by the Society of Vertebrate Paleontology
TEMPORAL CONGRUENCE AND THE ORIGIN OF BIRDS CHRISTOPHER A. BROCHU1 and MARK A. NORELL2, 1Department of Geology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, IL 60605, firstname.lastname@example.org; 2Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th St., New York, NY 10024
The preponderance of available character evidence indicates that birds evolved from coelurosaurian theropod dinosaurs (Ostrom, 1976; Gauthier, 1986; Holtz, 1994; Sereno, 1997; Padian and Chiappe, 1998; Forster et al., 1998; Makovicky and Sues, 1998). Critics of this perspective have been quick to point out potential problems based on assumptions of process from developmental biology, ecology, and functional expectations (Feduccia and Wild, 1993; Burke and Feduccia, 1997; Ruben et al., 1997; Feduccia and Martin, 1998). One often cited piece of counterevidence is the so-called ‘‘temporal paradox,’’ in which the closest hypothesized dinosaurian relatives of birds are said to appear later in the fossil record than do birds (Feduccia, 1996; Hou et al., 1996; Feduccia and Martin, 1998; Thomas and Garner, 1998). Here we show that the ‘‘temporal paradox’’ exists only as a local artifact caused by a failure to consider all relevant portions of the tree concurrently. When entire trees are empirically evaluated, stratigraphic data are most congruent with a hypothesis joining birds with theropod dinosaurs. The temporal paradox argument has been addressed philosophically (e.g., Padian and Chiappe, 1998). The temporal paradox argument assumes that stratigraphic order should overturn a well-corroborated phylogeny for a group with a fragmentary fossil record; it confuses the concepts of ‘‘ancestor’’ and ‘‘sister taxon’’; and it ignores or dismisses fragmentary remains that may close the stratigraphic gap between first birds and their closest nonavian theropod relatives (Jensen and Padian, 1989; Chure, 1994; Zhao and Xu, 1998). But despite these philosophical problems, the temporal paradox argument continues to appear in reviews of the issue. We thus decided to address it empirically. This gives us the opportunity to highlight the importance of quantitative approaches to addressing phylogenetic debates and the necessity of considering a whole tree rather than a single node in isolation. Several techniques have been proposed to assess how well a set of first appearance dates corresponds to the predictions of specific phylogenetic hypotheses, including the Stratigraphic Consistency Index (SCI; Huelsenbeck, 1994), the Sum of Minimum Implied Gaps (SMIG; Benton and Storrs, 1994, 1996; see also Fisher, 1991, and Weishampel, 1996, for identical statistics under different names), and the Manhattan Stratigraphic Measure (MSM; Siddall, 1998). The first measures how well the relative sequence of first appearances matches the expected sequence of appearances on a cladogram, while the others evaluate absolute temporal disparity. All metrics are applied globally over entire trees, rather than concentrating locally on a specific node. SCI is the ratio between the number of nodes consistent with stratigraphy and the total number of nodes on the tree. As SCI diminishes, fit diminishes. SCI is known to be biased toward pectinate trees (Siddall, 1996), but the different trees in this study can be considered isomorphic on the basis of Heard’s Imbalance Index (Table 2). SMIG is simply a measure of the sum length of minimum ghost lineages over the entire tree. Smaller SMIG’s indicate a better fit of stratigraphy with topology as lengths of ghost lineages are minimized. MSM considers a step matrix comprising the absolute difference between the first appearance dates of any two taxa in the tree. This matrix is optimized as a Sankoff character on two trees, the tree under analysis and a tree that optimizes differences on the matrix. In effect, one is treating the absolute difference between first appearances of taxa as a single multistate character and optimizing it using Wagner parsimony, with the absolute distances among first appearances acting as weights among different character state transformations. One divides the length of the minimal tree by that of the study tree to obtain MSM. Larger MSM ratios are indicative of
a better fit between stratigraphy and a topology. Although both SMIG and MSM will suffer from data set size biases, MSM is thought to avoid some of the problems in other methods with respect to nonindependence of ranked data from a hierarchical pattern by making pairwise comparisons between all ingroup taxa (Siddall, 1998). To assess the ‘‘temporal paradox’’ in bird origins, comparisons must be made between competing phylogenies. Unfortunately, the temporal paradox argument is invariably made in a phylogenetic vacuum. Stratigraphic shortcomings of a grouping of birds within Dinosauria are pointed out, but no competing phylogeny for the relationships of birds with other amniotes is proposed that better explains both the distribution of character states among ingroup taxa and their stratigraphic ranges. But we can construct alternatives by drawing Avialae adjacent to putative nondinosaurian relatives of birds as proposed in the literature (Fig. 1). The most common nondinosaurian ‘‘alternative’’ is the ‘‘thecodontian’’ hypothesis, which dates to the early part of the 20th Century (Heilmann, 1926). This is a very difficult hypothesis to characterize phylogenetically, because ‘‘Thecodontia’’ is a paraphyletic assemblage of virtually any archosauriform not obviously belonging to Crocodyliformes, Pterosauria, or Dinosauria (Gauthier and Padian, 1985). For this analysis, we considered several variants of the thecodontian hypothesis that could be constructed from the literature by making birds the sister group to Crocodylomorpha (Walker, 1972, 1977; Martin, 1991), Euparkeria (Welman, 1995), Scleromochlus (Feduccia, 1996), and Dinosauromorpha (Tarsitano and Hecht, 1980). We also considered the enigmatic archosauromorph reptile Megalancosaurus, which was regarded as a possible avian relative by Tarsitano (1991) and Feduccia and Wild (1993). These alternatives can be mapped onto a phylogeny for archosauromorph reptiles as shown in Figure 1. The base phylogeny is based on work by Gauthier et al. (1988), Sereno (1991), Sereno and Arcucci (1994a, b), Parrish (1993), Renesto (1994), and Merck (1997). Other alternatives have been proposed, such as Longisquama, Sharovipteryx, and Cosesaurus (see Padian and Chiappe, 1998, for review), but these involve problematic taxa of uncertain diapsid affinity. Because we could not construct alternative cladograms including them, they were not considered in this analysis. It should also be borne in mind that the alternatives we considered have generally been considered bird relatives and not ancestors per se. In fact, Feduccia (1996) regarded Longisquama, Megalancosaurus, and Scleromochlus as parts of a radiation that, in combination, preserved features in common with birds. We compare the stratigraphic implications these alternatives make against those of a placement of birds within Dinosauria (Fig. 1E). Minor differences remain among current analyses of theropod phylogeny, so four different topologies were analyzed (Fig. 1A–D). They are referred to as H (Holtz constraint; Holtz, 1994), S (Sereno constraint; Sereno, 1997), F (Forster constraint; Forster et al., 1998) and M (Makovicky and Sues constraint; Makovicky and Sues, 1998). A few taxa, such as Ornitholestes and Compsognathus, were not common to all four analyses, but were considered important both for their stratigraphic position in the Late Jurassic and their phylogenetic position within Theropoda. Hence, when missing from a tree, they were added by hand based on their placement in the other trees. A few problematic taxa were not included, such as Therezinosauroidea and the new feathered theropods (Ji et al., 1998) because they are not fully described, highly labile in recent analyses, and would greatly complicate subsequent calculations. Some authors (e.g., Gauthier, 1986; Brinkman and Sues, 1987; Novas,
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000
FIGURE 1. Constraint trees used in this study. A, dinosaur phylogeny based on Holtz (1994); B, dinosaur phylogeny based on Sereno (1997); C, dinosaur phylogeny based on Forster et al. (1998); D, dinosaur phylogeny based on Makovicky and Sues (1998); E, phylogeny of nondinosaurian archosauromorphs, with nondinosaurian placements for birds (Avialae) indicated. Trees simplified from Gauthier et al. (1988), Sereno (1991), Sereno and Arcucci (1994a, b), Parrish (1993), and Merck (1997). Because of computational limitations, six taxa considered in SCI and SMIG calculations (Compsognathus, Eoraptor, Herrerasauridae, Marasuchus, Pseudolagosuchus, and Choristodera) were not included in calculation of MSM.
1992; Padian and May, 1993) have suggested a position for Eoraptor and Herrerasauridae outside Dinosauria; alternate placements of these two taxa do not alter the results obtained here. Absolute ages for fossil taxa were used when available, but most often first appearances were calibrated to the midpoint of their first appearance stage. Absolute times were taken from Harland et al. (1990), and first appearances of taxa were obtained as shown in Table 1. In a few cases (e.g., Euryapsida, Lepidosauromorpha, Protorosauria), the precise stage of first appearance was not known, but a portion of a period (e.g., Early Triassic, Late Permian) could be specified. The midpoint of the given chronostratigraphic unit was used to calibrate the first appearance datum for each taxon, as indicated in Table 1. Calibrations were based as much as possible on nondental remains that unequivocally belong to their respective groups; we acknowledge that fragmentary remains are known that can draw several of these groups back in time (e.g., Jensen and Padian, 1989; Chure, 1994). Measures of relative (SCI) and absolute (SMIG, MSM) disparity are shown in Table 2 and, graphically, in Figure 2. SCI is essentially unable to distinguish among the different scenarios (Table 2, Figure 2). Most SCI values for nondinosaurian trees under the H or F constraints are equivalent to those of their respective dinosaurian trees, and with the S or M trees most are actually lower than the dinosaurian tree. Relative consistency increases only if the H or F trees are used, and if birds are close to either Euparkeria or Megalancosaurus, with an improvement of only one out of 35 nodes (Table 2). The node linking birds to their sister taxon is stratigraphically consistent under all nondinosaurian scenarios, but such scenarios often create new inconsistencies elsewhere in the tree that did not exist under the dinosaurian constraint. Hence, relative consistency is generally invariantâ€” local disparity may be eliminated, but net global disparity does decrease and may increase. The H tree (SMIG5407.6) matches the absolute temporal pattern better than the others, although the difference between it and either the S or M trees (SMIG5419.9) is very small (2.9%; Table 2). These trees
match stratigraphy better than the F tree (SMIG5440.6), with the H tree requiring 7.5% less missing record and the others 4.7%. The minimum ghost lineage separating birds from their nearest dinosaurian relative is short. Based on the presence of dromaeosaurids in the Barremian (Kirkland et al., 1993), the minimum ghost lineage is only 20.9 my long in the H, S, and M constraints; and based on the oldest troodontid non-dental remains in the Aptian (Barsbold et al., 1987; Russell and Dong, 1993), the minimum ghost lineage is 30.6 my under the F constraint. These contrast with the much longer ghost lineages of up to 75 my implied by some applications of the temporal paradox argument (e.g., Hinchliffe, 1997). All of the trees that exclude birds from Dinosauria had higher SMIG values than their respective dinosaurian trees (Table 2). SMIG ranged from 437.9 (birds 1 Crocodyliformes, S constraint) to 494.2 (birds 1 Euparkeria, M constraint). Regardless of the theropod tree used, a crocodyliform relationship implied less missing record, and a relationship to Euparkeria implied the most. Changes from dinosaurian to non-dinosaurian scenarios were most dramatic under the H constraint (10.87% to 16.10%) and least under the F constraint (1.85% to 7.71%). Comparison of MSM values generally corroborates the results observed with SMIG. The H and S trees (MSM50.467) had slightly higher values than the F or M trees (MSM50.438). Trees excluding birds from Dinosauria have lower MSM values (Table 2). MSM for non-dinosaurian scenarios was slightly higher under the S constraint and lower under the F constraint, which was identical to the M constraint in all cases. Although there is a coarse relationship between SMIG and MSM, they are not directly correlated. Relationships with Crocodyliformes fared better under both SMIG and MSM, but whereas the highest SMIG were associated with a relationship to Euparkeria, a relationship with Megalancosaurus yielded a slightly lower MSM than one with Euparkeria. This may result from the pruning of taxa from the MSM analysis such that the two numbers are not directly comparable in this study. From the data presented in Table 2 and Figure 2, there is no empirical justification for applying stratigraphic arguments against a dinosaurian
TABLE 1. Calibrations for taxa considered in this study. Taxon Avialae Dromaeosauridae Troodontidae Oviraptorosauria Tyrannosauridae Ornithomimosauria Ornitholestes Allosauroidea Megalosauroidea Ceratosauria Eoraptor Herrerasauridae Sauropodomorpha Ornithischia Marasuchus Pseudolagosuchus Lagerpeton Scleromochlus Pterosauria Ornithosuchidae Parasuchia Prestosuchus Aetosauria Rauisuchia Sphenosuchia Cocodyliformes Euparkeria Proterochampsidae Erythrosuchidae Proterosuchidae Trilophosauridae Proterosauridae Choristodera Rhynchosauria Megalancosaurus Euryapsida Lepidosauromorpha
148.9 128.2 118.3 112.0 97.0 137.9 148.9 182.5 163.7 229.2 227.8 227.8 216.5 229.2 237.3 237.3 237.3 229.2 229.2 229.2 229.2 237.3 229.2 237.3 229.2 208.8 243.1 237.3 242.7 237.3 229.2 250.5 216.5 243.1 216.5 243.1 250.5
Sereno, 1997 Sereno, 1997 Sereno, 1997 Makovicky and Sues, 1998 Sereno, 1997 Sereno, 1997 Sereno, 1997 Sereno, 1997 Sereno, 1997 Sereno, 1997 Rogers et al., 1993 Rogers et al., 1993 Sereno, 1997 Sereno, 1997 Benton, 1995 Benton, 1995 Benton, 1995 Sereno, 1991 Sereno, 1991 Sereno, 1991 Parrish, 1993 Parrish, 1993 Parrish, 1993 Parrish, 1993 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Benton, 1995 Merck, 1997 Merck, 1997 Benton, 1990
TABLE 2. Imbalance Index (Im), Stratigraphic Consistency Index (SCI), Sum of Minimum Implied Gaps (SMIG), and Manhattan Stratigraphic Measure (MSM) for different placements of birds on different dinosaur phylogenies (H, S, F, and M constraints). Expected Im for a tree of 38 taxa is 0.1071. Taxon H
Theropod ‘‘Thecodont’’ Scleromochlus Croc.morph Croc.form Euparkeria Megalanc. Theropod ‘‘Thecodont’’ Scleromochlus Croc.morph Croc.form Euparkeria Megalanc. Theropod ‘‘Thecodont’’ Scleromochlus Croc.morph Croc.form Euparkeria Megalanc. Theropod ‘‘Thecodont’’ Scleromochlus Croc.morph Croc.form Euparkeria Megalanc.
0.668 0.677 0.652 0.656 0.656 0.647 0.638 0.677 0.677 0.652 0.656 0.656 0.647 0.638 0.677 0.677 0.652 0.656 0.656 0.647 0.638 0.677 0.677 0.652 0.656 0.656 0.647 0.638
0.556 0.556 0.556 0.556 0.556 0.583 0.583 0.583 0.556 0.556 0.556 0.556 0.583 0.583 0.556 0.556 0.556 0.556 0.556 0.583 0.583 0.556 0.528 0.528 0.528 0.528 0.556 0.556
407.6 485.8 477.7 477.7 457.3 491.6 465.0 419.9 466.4 458.3 458.3 437.9 472.2 445.6 440.6 477.4 469.3 469.3 448.9 483.2 456.6 419.9 488.4 480.3 480.3 459.9 494.2 467.6
0.466 0.394 0.394 0.394 0.416 0.386 0.381 0.467 0.405 0.405 0.405 0.426 0.396 0.390 0.438 0.374 0.374 0.374 0.394 0.367 0.362 0.438 0.374 0.374 0.374 0.394 0.367 0.362
FIGURE 2. SCI, SMIG, and MSM for the Holtz (H), Sereno (S), Forster et al. (F), and Makovicky and Sues (M) constraints for various placements of birds within Archosauromorpha (see Table 2). Competing placements of Avialae are indicated on the X axis; values for each constraint are indicated by capital letters (H, S, F, and M) at each placement. When capital letters are adjacent to each other, indicated constraints have identical values for that placement.
hypothesis for bird origins. Philosophical problems aside, a ‘‘temporal paradox’’ exists only if a single node is examined in isolation from the rest of the tree. Global relative stratigraphic consistency is uniform among all hypotheses, and measures of global absolute disparity strongly favor birds as dinosaurs. Moreover, the magnitude of the ghost lineage on the dinosaurian sister group to birds is much shorter than sometimes reported. Moving birds out of Dinosauria may increase global absolute disparity by as much as 15 percent, with most of the added stratigraphic debt resulting from a longer ghost lineage for birds. We thus conclude that stratigraphy presents no significant paradox to the congruent hypotheses supported by multiple independent character data sets. Acknowledgments—We thank P. Wagner, M. Siddall, and J. Merck for fruitful discussion and computational assistance. M. Novacek, C. Sumrall, and K. Padian reviewed earlier versions of this paper. This work was supported by the Department of Vertebrate Paleontology at the American Museum of Natural History and NSF DEB 9407999 (MAN).
JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000 LITERATURE CITED
Barsbold, R., H. Osmo´lska, and S. M. Kurzanov. 1987. On a new troodontid (Dinosauria, Theropoda) from the Early Cretaceous of Mongolia. Acta Palaeontologica Polonica 32:121–132. Benton, M. J. 1990. Phylogeny of the major tetrapod groups: morphological data and divergence dates. Journal of Molecular Evolution 30:409–424. 1995. Late Triassic to Middle Jurassic extinctions among continental tetrapods: testing the pattern; pp. 366–397 in N. C. Fraser and H. D. Sues (eds.), In the Shadow of the Dinosaurs: Early Mesozoic Tetrapods, Cambridge University Press, New York. , and G. W. Storrs. 1994. Testing the quality of the fossil record: paleontological knowledge is improving. Geology 22(2):111–114. , and 1996. Diversity in the past: comparing cladistic phylogenies and stratigraphy; pp. 19–40 in M. E. Hochberg and R. Barbault (eds.), Aspects of the Genesis and Maintainance of Biological Diversity, Oxford University Press, New York. Brinkman, D. B., and H.-D. Sues. 1987. A staurikosaurid dinosaur from the Upper Triassic Ischigualasto Formation of Argentina and the relationships of the Staurikosauridae. Palaeontology 30:493–503. Burke, A. C., and A. Feduccia. 1997. Developmental patterns and the identification of homologies in the avian hand. Science 278:666– 668. Chure, D. J. 1994. Koparion douglassi, a new dinosaur from the Morrison Formation (Upper Jurassic) of Dinosaur National Monument; the oldest troodontid (Theropoda: Maniraptora). Brigham Young University Geology Studies 40:11–15. Feduccia, A. 1996. The Origin and Evolution of Birds. Yale University Press, New Haven, 420 pp. , and R. Wild. 1993. Birdlike characters in the Triassic archosaur Megalancosaurus. Naturwissenschaften 80:564–566. , and L. D. Martin. 1998. Theropod–bird link reconsidered. Nature, 391:754. Fisher, D. C. 1991. Phylogenetic analysis and its implication in evolutionary paleobiology; pp. 103–122 in N. L. Gilinsky and P. W. Signor (eds.), Short Courses in Paleontology 4: Analytical Paleobiology. Paleontological Society, Knoxville, TN. Forster, C. A., S. D. Sampson, L. M. Chiappe, and D. W. Krause. 1998. The theropod ancestry of birds: new evidence from the Late Cretaceous of Madagascar. Science 279:1915–1919. Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds. Memoirs of the California Academy of Sciences 8:1–55. , and K. Padian. 1985. Phylogenetic, functional, and aerodynamic analyses of the origin of birds and their flight; pp. 185–197 in M. K. Hecht, J. H. Ostrom, G. Viohl, and P. Wellnhofer (eds.), The Beginnings of Birds, Freunde des Jura-Museum, Eichstatt. , A. G. Kluge, and T. Rowe. 1988. Amniote phylogeny and the importance of fossils. Cladistics 4:105–209. Harland, W. B., R. L. Armstrong, A. V. Cox, L. E. Craig, A. G. Smith, and D. G. Smith. 1990. A Geologic Time Scale 1989. Cambridge University Press, New York, 263 pp. Heilmann, G. 1926. The Origin of Birds. Appleton, New York, 210 pp. Holtz, T. R. 1994. The phylogenetic position of the Tyrannosauridae: implications for theropod systematics. Journal of Paleontology 65: 1100–1117. Hou, L., L. D. Martin, Z. Zhou, and A. Feduccia. 1996. Early adaptive radiation of birds: evidence from fossils from northeastern China. Science 274:1164–1167. Huelsenbeck, J. P. 1994. Comparing the stratigraphic record to estimates of phylogeny. Paleobiology 20:470–483. Jensen, J. A., and K. Padian. 1989. Small pterosaurs and dinosaurs from the Uncompahgre Fauna (Brushy Basin Member, Morrison Formation, ?Tithonian), Late Jurassic, western Colorado. Journal of Paleontology 63:364–373. Ji, Q., P. J. Currie, M. A. Norell, and S.-A. Ji. 1998. Two feathered dinosaurs from northeastern China. Nature 393:753–761. Kirkland, J. I., R. Gaston, and D. Burge. 1993. A large dromaeosaur (Theropoda) from the Lower Cretaceous of eastern Utah. Hunteria 2(10):1–16. Makovicky, P. J., and H. D. Sues. 1998. Anatomy and phylogenetic relationships of the theropod dinosaur Microvenator celer from the
Lower Cretaceous of Montana. American Museum Novitates 3240: 1–27. Martin, L. D. 1991. Mesozoic birds and the origin of birds; pp. 485– 540 in H-P. Schultze and L. Trueb (eds.), Origins of the Higher Groups of Tetrapods, Cornell University Press, Ithaca, New York. Merck, J. W. 1997. A phylogenetic analysis of the euryapsid reptiles. Ph.D. dissertation, University of Texas at Austin, 785 pp. Novas, F. E. 1992. Phylogenetic relationships of basal dinosaurs, the Herrerasauridae. Palaeontology 35:51–62. Ostrom, J. H. 1976. Archaeopteryx and the origin of birds. Biological Journal of the Linnean Society 8:91–182. Padian, K., and L. M. Chiappe. 1998. The origin and early evolution of birds. Biological Reviews 73:1–42. , and C. L. May. 1993. The earliest dinosaurs; pp. 379–381 in S. G. Lucas and M. Morales (eds.), The Nonmarine Triassic, New Mexico Museum of Natural History and Science Bulletin 3. Parrish, J. M. 1993. Phylogeny of the Crocodylotarsi, with reference to archosaurian and crurotarsan monophyly. Journal of Vertebrate Paleontology 13(3):287–308. Renesto, S. 1994. Megalancosaurus, a possibly arboreal archosauromorph (Reptilia) from the Upper Triassic of northern Italy. Journal of Vertebrate Paleontology 14:38–52. Rogers, R. R., C. C. Swisher III, P. C. Sereno, A. M. Monetta, C. A. Forster, and R. N. Martinez. 1993. The Ischigualasto tetrapod assemblage (Late Triassic, Argentina) and 40Ar/39Ar dating of dinosaur origins. Science 260:794–797. Ruben, J. A., T. D. Jones, N. R. Geist, and W. J. Hillenius. 1997. Lung structure and ventilation in theropod dinosaurs and early birds. Science 278:1267–1270. Russell, D. A., and Z.-M. Dong. 1993. A nearly complete skeleton of a new troodontid dinosaur from the Early Cretaceous of the Ordos Basin, Inner Mongolia, People’s Republic of China. Canadian Journal of Earth Sciences 30:2163–2173. Sereno, P. C. 1991. Basal archosaurs: phylogenetic relationships and functional implications. Society of Vertebrate Paleontology Memoir 2:1–53. 1997. The origin and evolution of dinosaurs. Annual Review of Earth and Planetary Sciences 25:435–489. , and A. B. Arcucci. 1994a. Dinosaur precursors from the Middle Triassic of Argentina: Lagerpeton chanarensis. Journal of Vertebrate Paleontology 13:385–399. , and 1994b. Dinosaur precursors from the Middle Triassic of Argentina: Marasuchus lilloensis gen. nov. Journal of Vertebrate Paleontology 14:53–73. Siddall, M. E. 1996. Stratigraphic consistency and the shape of things. Systematic Biology 45(1):111–115. 1998. Stratigraphic fit to phylogenies: A proposed solution. Cladistics 14:201–208. Tarsitano. 1991. Archaeopteryx: Quo vadis? pp. 541–576 in H.-P. Schultze and L. Trueb (eds.), Origins of the Higher Groups of Tetrapods, Cornell University Press, Ithaca, New York. , and M. K. Hecht. 1980. A reconsideration of the reptilian relationships of Archaeopteryx. Zoological Journal of the Linnean Society 69:149–182. Thomas, A. L. R., and J. P. Garner. 1998. Are birds dinosaurs? Trends in Ecology and Evolution, 13:129–130. Walker, A. D. 1972. New light on the origin of birds and crocodiles. Nature 237:257–263. 1977. Evolution of the pelvis in birds and dinosaurs; pp. 319– 357 in S. M. Andrews, R. S. Miles, and A. D. Walker (eds.), Problem in Vertebrate Evolution, Academic Press, New York. Weishampel, D. B. 1996. Fossils, phylogeny, and discovery: a cladistic study of the history of tree topologies and ghost lineage durations. Journal of Vertebrate Paleontology 16:191–197. Welman, J. 1995. Euparkeria and the origin of birds. South African Journal of Science 91:533–537. Whetstone, K. N., and P. J. Whybrow. 1983. A ‘‘cursorial’’ crocodilian from the Triassic of Lesotho (Basutoland), southern Africa. Occasional Papers of the Museum of Natural History, The University of Kansas 106:1–37. Zhao, X., and X. Xu. 1998. The oldest coelurosaurian. Nature 394:234– 235. Received 24 February 1999; accepted 11 May 1999.